Conventional ferrous, powder metal parts produced by simple pressing and sintering are known to have rather low dynamic properties; that is, impact and fatigue strength, because of the presence of the pores in such parts. Approaches to overcome this drawback include various methods for achieving full or nearly full density.
One of the least expensive methods to achieve nearly full density is to infiltrate such parts with copper or a copper based infiltrant. Infiltration of porous iron and steel parts with copper has been in commercial use since the 1940's. The most common reason for using this process is to improve the mechanical properties of a powder metallurgy part.
In spite of the ability to achieve nearly full density by infiltration with a suitable infiltrant, published data on copper infiltrated ferrous powder metal parts shows only small improvement in dynamic properties over uninfiltrated parts.
The impact strength of powder metal parts is important for many end use applications. One example is gear parts. A critical area of a gear part is at the root of the gear teeth, and a weakness in this area creates a potential for gear failure. In determining impact strength on gear teeth, a special tool applies a tangential force to a gear tooth, and the impact strength is essentially the energy necessary to establish failure in the gear critical area.
Another example of a powder metal part in which impact strength is important, is the hammer used in a hammer-type mill, such as found in a garbage disposal unit. A plurality of hammers are secured to a rotor by means of bolts. The hammer is provided with a slotted shank, in which the securing bolt slides, and a hammer head. The critical area is that area between the head and shank, and as with gear teeth, an imperfection in the critical area creates a potential for failure.
The impact strength for these hammers is determined by subjecting the hammer shanks to a side-directed moment of force and, here also, the energy necessary to establish failure is essentially the impact strength.
A conventional method for determining impact strength of specimens is the Charpy impact test procedure described in the Metal Powder Industries Federation (MPIF) Standard 40, 1974 Metal Powder Industries Federation P.O. Box. 2054, Princeton, NJ 08540. In this test, unnotched specimens are formed into a defined rectangular shape having specified dimensions, and are placed in a pendulum-type impact machine with a capacity of at least 110 foot pounds (15.2 m-kg). The impact strength is the average of three tests reported to the nearest foot pound. Standard 40 is incorporated by reference herein. For purposes of the present application, the term impact strength, where used herein, shall mean, unless otherwise noted, the strength values obtained following the Charpy-type test procedure outlined in Standard 40.
Another mechanical property of interest in the preparation of many ferrous powder metal parts is the tensile strength. This property, and the test for determining it, are described in MPIF Standard 10, also incorporated by reference herein. An aspect of the tensile strength of a powder metal part is the elongation of the part that occurs prior to failure. In the present application, the tensile strength and elongation shall be given (unless otherwise stated) in terms of kips per square inch (ksi) and percent elongation (E%), respectively, following the procedure of Standard 10.
In the following Table 1, tensile strength data and impact toughness of typical powder metal parts, determined by the above MPIF tests are given. As can be appreciated from the data of Table 1, the impact strength improvement possible with copper infiltration is limited. Unnotched Charpy impact values range from 3 to 35 foot pounds for iron/carbon steels, less than 2 to 8 foot pounds for copper/iron steels, and only about 5 to about 25 foot pounds for copper infiltrated steels. These values represent the present state-of-the-art for powder metal parts. Also of interest in the data of Table 1 is the fact that, as a general rule, if the impact strength is increased the tensile strength tends to be less.
TABLE 1 __________________________________________________________________________ TENSILE AND IMPACT DATA FOR P/M PARTS TYPE OF P/M COMPOSITION PART/MPIF STD. Fe C.sup.1 Cu UTS/KSI E % IMPACT - FT LBS __________________________________________________________________________ Iron-Carbon F-0000-10 97.7-100 0-0.3 18 1.5 3 F-0000-20 38 7.0 35 F-3008-30 97.1-99.4 .6-.9 42 1 4 Copper-Steels FC-0200-24 93.8-98.5 0-0.3 1.5-3.9 35 1 6 FC-0200-90 93.8-98 0-0.3 1.5-3.9 100 1 5.9 Copper Infiltrated Steels FX-1008-50 82.2-91.4 .6-.9 8-14.9 87 3.0 10 FX-1008-110 HT.sup.2 82.2-91.4 .6-.9 8-14.9 120 0.5 7 FX-1000-25 82.8-92 0-0.3 8-14.9 51 7 25 FX-2000-25 72.7-85 0-0.3 15-25 46 3.0 15 FX-2008-60 72.1-84.4 .6-.9 15-25 80 1 7 FX-2008-90 HT 72.1-84.4 .6-.9 15-25 100 0.5 5 Ref. 1 108 13.5 46 __________________________________________________________________________ .sup.1 % carbon in steel matrix .sup.2 heat treated
Reference 1 in the above table is a paper published in 1949, by R. Kieffer and F. Benesovsky, entitled "The Production and Properties of Novel Sintered Alloys (Infiltrated Alloys)", Berg- und Huttenmannische Monatshefte, Volume 94 (No. 8/9), 1949, pages 284-294. The paper reports that an impact strength of about 46 foot pounds can be obtained by sintering and infiltrating under hydrogen and then heat treating the infiltrated parts. However, even this figure is low.
U.S. Pat. No. 2,768,917 to Pettibone, dated Oct. 30, 1956, also discloses a two-step sintering and infiltrating process under hydrogen atmosphere for ferrous metal parts using a copper alloy infiltrant. On this patent no impact strength data is given.
Considering that copper infiltrated parts are nearly full dense structures, it is somewhat surprising that impact values of commercial copper infiltrated parts are generally less than about 15 foot pounds.
It is known to increase impact strength by the use of alloying procedures, or increasing the density of a part through double pressing and sintering, or by hot pressing or powder forging. All these processes are more expensive, particularly if they require the handling of hot compacts, presses to deform the parts, and expensive dies. Also, as reported by Rostoker and Clemens in The International Journal of Powder Metallurgy and Powder Technology, Volume 17, No. 4, 1981, pages 278-289/280, excessive reduction in pore size is undesirable, the initial sintered density level representing a compromise between the necessity for interconnection between voids and undesirably excessive void volume.
In a procedure reported by Rostoker and Clemens, on page 280, minus 200 mesh iron powder specimens were compacted in one inch diameter cylinders to a green density of 6.55-6.60 grams/cm.sup.3. The specimens were dewaxed for 2.5 hours at 625.degree. C. with nitrogen, and then sintered in vacuum (1.times.10.sup.-4 torr) at 1150.degree. C., for one hour. Infiltration with a copper manganese alloy (e.g., 14% Mn) was then carried out under vacuum at 1100.degree. C. for three hours. There is no reference in the article to impact strength, nor is there any suggestion that the use of this procedure could give improved impact strength. In fact, the paper reports, on page 285, ". . . In all cases general yielding (a resolvable yield stress) is absent before fracture. Achievement of a yield stress as well as high tensile strength is an important objective which has not been realized . . . ", implying very low impact strength. Suggested possible causes given were a degradation of the toughness of the steel matrix, residual stresses, and a weak interface between the steel and copper alloy.
In recent years, several other investigators have also attempted to utilize the well developed theories of liquid metal infiltration for the purpose of raising the mechanical properties of copper infiltrated parts.
By way of example, it is described in a paper by Ashurst et al "Copper infiltration of steel: Properties and Applications," Progress in Powder Metallurgy, (H. S. Nayar et al, editors), volume 39, pages 163-182, that impact toughness of copper infiltrated steels can be increased up to about 30 foot pounds through control of erosion. This was minimized through proper metallurgical formulation of the copper infiltrant, and strategic placement of the infiltrant slug onto the steel part.
In 1973, Kimura & Hamamoto, in a publication entitled "Strengthening of Iron and Powder Compacts by Infiltration", Modern Developments in Powder Metallurgy, (H. H. Hausner and W. E. Smith, Eds.), 1973, pp. 135-147, the low mechanical properties of copper infiltrated parts are attributed to the great discrepancy in strength between the steel matrix and the copper phase.